Get you an accurate wedge. Itís a rubber piece that fits in between upper and lower and snugs everything up. Unless the upper and lower were made for each other as a matched set you will almost always have a little play.

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the upper and lower are solid when together it's just right where the bolt carrier meets the buffer assembly there is a gap between the the upper receiver housing and the lower receiver housing...the 2 bolt and buffer touch there is just light that can be seen through the 2 housings...